Up to now, there have been commonly used, in the reducers of construction machines and earth-moving machines, gears produced by applying carburizing/quenching or carbonitriding/quenching to Scr-based, SCM-based or SNCM-based low carbon steel, since high contact fatigue strength (no less than 200 kgf/mm2) is considered to be an important factor. Some gears used under comparatively low interface pressure (up to 150 kgf/mm2) are produced by applying thermal treatment such as induction hardening to steel having a carbon concentration of 0.35 to 0.55 wt % and suited for induction hardening (see “Heat Treatment for Steel” (revised edition No. 5, p110, Tables 2-38, 2-39) edited by The Iron and Steel Institute of Japan, Publication Office: Maruzen Co., Ltd.).
Reducers employed in construction machines and earth-moving machines require less expensive gears having higher strength and higher resistance to interface pressure, because of the recent tendency to higher output power and compactness.
Construction machines and earth-moving machines often stride obstacles such as rocks and structures during traveling and break up such obstacles while making a turn. Therefore, the gears of the reducer used for running and turning the vehicle body receive impulsive load. Under such a condition, carburized/quenched gears are susceptible to damage.
Induction hardened gears have higher toughness than carburized/quenched gears, but where induction hardened gears are used under a high interface pressure of 150 kgf/mm2 more as noted earlier, defects in terms of contact fatigue strength (e.g., pitting, scuffing and premature wear of tooth flanks) are likely to occur.
The invention is directed to overcoming the above problems and therefore the invention aims to reduce local adhesion occurring during the rolling and sliding movement of a gear by dispersing undissolved fine cementite in austenite through induction hardening of the rolling contact surface. This idea is conceived from the fact that the temperature of the tooth flanks of a gear used under a rolling/sliding condition increases up to 300° C. owing to wear and heat generation caused by local adhesion that occurs in a boundary-lubrication condition. The invention also aims to provide an inexpensive rolling element for use under high interface pressure such as induction hardened gears, the rolling element having a temper hardness of HRC 50 or more at 300° C. Such a rolling element is produced from a steel material containing a large amount of Al and/or Si which effectively enhances temper softening resistance in low-temperature tempering at 300° C. Another object of the invention is to improve the seizure resistance of the tooth flanks of a gear by dispersing, into the tooth flanks beforehand, 0.1 to 0.5% by volume one or more kinds of compounds selected from the carbides, nitrides and carbonitrides of Ti, Zr, Nb, Ta and Hf which hardly solid-dissolve in austenite. A further object of the invention is to provide a rolling element exerting high toughness even in a highly hardened condition and the producing method thereof, the rolling element being achieved by more adequate co-addition of Al and Ni.